Caption: MODEL RELEASED. Baker's cyst (lump, centre) on the back of a 73-year-old woman's knee. This is a recurrence of a previous cyst. Also known as a popliteal cyst, this cyst occurs behind the knee and is an accumulation of fluid causing swelling. It is arthritic in nature in adults, due to degeneration of the joint. Treatment depends on the size. Small Baker's cysts can clear up without treatment.